I just don't understand from an evolutionary, anthropological, medical or whatever other scientific point of view, why it happens.... Doesn't make any sense to me whatsoever.....

I think when I can understand the why, it's easier to cope. But when it seems to come from nowhere, I just don't understand. She really is happy most of the time, although now that she's more alert she's sleeping less during the day and just wants to be held and interacted with... Which... (And I feel guilty for thinking this, cause she is SO wanted...)... Is quite tiresome... There are so many other things I need to get done as well as look after her. It's just not practical to be at her beck and call....
I have heard explanations for it, I doubt their accuracy because no one really knows why it happens but sometimes having a half baked theory is better than nothing at all......

The theories I have heard are related to wind. One is that breast milk has something in it that helps get wind out, but the level of this mysterious 'thing' is higher in milk produced at night - and peeks around 2-3am so by the evening the level of 'it' is pretty low thus babies get very uncomfortable with wind and at that age they just don't know what to do with themselves when they are uncomfortable so it all just turns pear shaped. The other theory is about wind kind of 'building up' during the day, they get rid of some of it but they get a bit of a back log by the end of the day and it causes discomfort and again - pear shaped. Compound this with little bodies often being veeeery tired at the end of the day and making discomfort harder to deal with - double pear shaped. Compound that with a wonder week at 12 weeks and you guessed it, triple pear shaped.

I think you've said she is 3 months? I remember in the book Baby Love says that this crying in the evening stage typically - but not always - goes from roughly 5-6 weeks, peaks at about 8 weeks and lasts until about 12 weeks, but I found it was more like 14 weeks. But the good news is that it does end! I totally remember being exactly where you are with my first and when you are in the midst of it, it feels like it is going to be like this forever and you will always have this crying poppet in the evenings. But again the good news is everything is a stage, everything passes, everything has an ebb and flo and you are really just waiting for the flo, not dealing with a long term issue.

I understand what you are saying about her not being at her beck and call, but I think a really good rule of thumb is that for the first 12 weeks (or maybe 14 if this stage lasts just that bit longer ) you just do whatever you have to do to get through the day. Some days that will be being at her beck and call but repeat message from earlier everything is a stage, everything passes, everything has an ebb and flo. Around 12 weeks these beautiful creatures go through their wonder week and it is a really tough time but afterwards there is more calm. Not saying it all turns to beer and skittles then but it changes and that change is for the better. Being at her beck and call for weeks at a time is easier when you know it is a short sprint and not a long haul. It makes life difficult/impossible for a time but then it passes and your little one will have learnt something new like how to laugh and then she will amaze you even more that she did yesterday.